Humanity is a Virus

"Humanity is a virus." - almost Agent Smith
Cultures are born, spread, wane and die. Virus' are the same. Cultures thereforce have some of the same properties as virus' For a culture to be successful, the number of people-groups adopting the culture must be greater than the number of people groups abandoning the culture. If a culture belongs to exactly one people group it is fragile. Successful cultures spread.

Here's where it gets meta: Successful cultures influence the people group to encourage their spread:

The British tried to enforce imperial rule on the world during colonial times.

The US tried to enforce democracy on the world in modern times.

Why? Because in order for the culture to reach the level of "controlling a nation" it had already evolved to be able to successfully "infect" many many people groups, and it was "trying" to infecting more.

So there we have it. Humans may not be a virus, but perhaps humanity is.